The lamination of substrates, such as documents, book covers, greeting cards, posters, postcards, brochure, catalogues, and identification cards, in a transparent plastics film is a promising market for polymer producers. Typically, the laminating material is an oriented polypropylene film or an oriented polyester film. In the past, laminating of documents with polymer films was achieved using solvent or water-based adhesives. However, adhesive lamination tends to be slow particularly with water-based adhesives, and, where solvents are involved, can raise environmental concerns.
More recently, therefore, focus has turned to alternative methods of lamination, such as extrusion coating, extrusion lamination and thermal lamination. In extrusion coating a bonding layer is extrusion coated onto a base film. In the document plastification application, the resulting composite film is thermally laminated onto the substrate by the application of heat and pressure. However, existing methods of extrusion coating generally require a primer to be applied to the base film to increase the strength of the bond between the bonding layer and the base film. There is therefore significant interest in the development of lamination systems that will operate without the use of primers. There is also interest in developing extrusion coating compositions that require lower temperatures and pressures in the thermal lamination step.
In our co-pending U.S. Provisional Application No. 61/227,997, filed Jul. 23, 2009, we have described a composite film comprising (a) a substrate layer and (b) a coating disposed on at least one side of the substrate, wherein the coating comprises a propylene copolymer which comprises units derived from propylene and from about 3 to about 25 wt % of units derived from ethylene and/or a C4 to C8 alpha-olefin and which has a melting temperature less than about 105° C. and a heat of fusion less than about 75 J/g. The polypropylene-based coating can be extrusion coated and/or laminated onto the substrate layer and exhibits good adhesion to the substrate layer without the need for a primer layer.
According to the present invention there is now provided a composition and process for producing a thermally laminated coating on a substrate, especially a document, which again can achieve a strong bond with the substrate without the use of a primer in the laminated coating. In addition, thermal lamination can generally be conducted at lower temperatures and pressures than with conventional lamination materials.